Backscattering ambient ism band signals

ABSTRACT

A backscatter tag device includes, in part, a receiver configured to receive a packet conforming to a communication protocol defining a multitude of codewords, a codeword translator configured to translate at least a first subset of the multitude of codewords disposed in the packet to a second multitude of codewords defined by the protocol in response to a data the backscatter tag is invoked to transmit, and a transmitter configured to transmit the packet supplied by the codeword translator at a frequency different than the first frequency at which the packer is received. The communication protocol may optionally be the 802.11 g/n, ZigBee or the Bluetooth communication protocol.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a US national stage of PCT/US2017/058371, filed Oct. 25, 2017, which claims the benefit under 35 USC 119(e) of U.S. Application Ser. No. 62/412,712, filed Oct. 25, 2016, entitled “Freerider: Backscattering Ambient ISM Band Signals”, the content each of which is incorporated herein by reference in its entirety for all purposes.

The present invention is related to application Ser. No. 15/676,474, filed Aug. 14, 2017, the content of which is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The present invention relates to communication systems and methods, and more particularly to a low power WiFi backscattering communication system and method.

BACKGROUND OF THE INVENTION

Backscatter communication has attracted interest for applications such as implantable sensors, wearables, and smart home sensing because of its ability to offer low power connectivity to these sensors. Such applications have severe power constraints. Implantable sensors for example have to last for years, while even more traditional smart home monitoring applications may benefit from sensors and actuators that can last several years. Backscatter communication can satisfy the connectivity requirements while consuming such low power as to be energized by harvesting energy, or with batteries that could last several years.

Current backscatter systems require specialized hardware to generate the excitation RF signals that backscatter radios can reflect, as well as to decode the backscattered signals. Recent research such as Wi-Fi backscatter to BackFi and passive WiFi have reduced the need for specialized hardware. Passive WiFi for example can decode using standard WiFi radios, however it still requires a dedicated continuous wave signal generator as the excitation RF signal source. BackFi needs a proprietary full duplex hardware add-on to WiFi radios to enable backscatter communication. Consequently, a need continues to exist for a backscatter system that can be deployed using commodity devices such as access points, smartphones, watches and tablets.

BRIEF DESCRIPTION OF THE INVENTION

FIG. 1 is a simplified view of a backscatter communication system 100, in accordance with one embodiment of the present invention.

FIG. 2 shows a number of OFDM symbols modulated on different subcarriers, as known in the prior art.

FIG. 3 shows the durations of a number of packets collected on a channel.

FIG. 4 shows the rate of decoding success as a function of distance, in accordance with one exemplary embodiment of the present invention.

FIG. 5 is a simplified high-level block diagram of a backscatter tag, in accordance with one embodiment of the present invention.

FIG. 6 shows various block diagrams of an 802.11 g/n transmission and reception blocks

FIG. 7 is a more detailed view of the scrambler shown in FIG. 6.

FIG. 8 shows the frequency spectrum of a backscattered Bluetooth signal.

FIG. 9A shows an experimental setup for testing a backscatter tag deployed in a line-of-sight, in accordance with one embodiment of the present invention.

FIG. 9B shows an experimental setup for testing a backscatter deployed in a non-line-of sight setup, in accordance with one embodiment of the present invention.

FIG. 10A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in a LOS deployment.

FIG. 10B shows the bit-error rate of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in a LOS deployment.

FIG. 10C shows the received signal strength indicator of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in a LOS deployment.

FIG. 11A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in an NLOS deployment.

FIG. 11B shows the bit-error rate of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in an NLOS deployment.

FIG. 11C shows the received signal strength indicator of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in an NLOS deployment.

FIG. 12A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and a ZigBee receiver.

FIG. 12B shows the bit-error rate of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and a ZigBee receiver.

FIG. 12C shows the received signal strength indicator of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and a ZigBee receiver.

FIG. 13A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and a Bluetooth receiver.

FIG. 13B shows the bit-error rate of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and a Bluetooth receiver.

FIG. 13C shows the received signal strength indicator of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and a Bluetooth receiver.

FIG. 14 shows the effect of the distance between the tag, in accordance with embodiments of the present, and the transmitter for WiFi 802.11 g/n, ZigbBee and Bluetooth communications protocols, in accordance with one embodiment of the present invention.

FIG. 15 shows the throughput of a WiFi 802.11 g/n when the backscatter tag is either present or absent, in accordance with one embodiment of the present invention.

FIG. 16A shows the backscatter tag throughput when an 802.11g/n WiFi is used as the excitation signal for the backscatter tag, in accordance with one embodiment of the present invention.

FIG. 16B shows the backscatter tag throughput when a ZigBee is used as the excitation signal for the backscatter tag with the tag, in accordance with one embodiment of the present invention.

FIG. 16C shows the backscatter tag throughput when a Bluetooth is used as the excitation signal for the backscatter tag with the tag, in accordance with one embodiment of the present invention.

FIG. 17A shows the aggregated throughput when respectively 4, 8, 12, 16, and 20 tags are positioned in the path of the transmitter, in accordance with one embodiment of the present invention.

FIG. 17B shows the Jain's fairness index when respectively 4, 8, 12, 16, and 20 tags are positioned in the path of the transmitter, in accordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention provide a system and method of communication that is complaint with an existing communications protocol, such as WiFi 802.11g/n, Bluetooth, and ZigBee, by backscattering another compliant packet and modulating its data on the resulting packet by codeword translation. According to some embodiments, applications can be built on existing wireless devices carrying such packets. A low-power backscatter communications system (hereinafter alternatively referred to as backscatter tag, or tag) is configured, in part, to receive a valid codeword disposed in the transmitted, for example, 802.11 g/n packet and translate it to a different valid codeword from, for example, the 802.11 g/n codebook. The specific translation encodes the bit that the backscatter tag seeks to transmit. The backscattered packet is therefore like any other, for example, 802.11g/n packet, albeit with a sequence of translated codewords depending on the data that backscatter tag seeks to communicate. Consequently it can be decoded by any standard 802.11g/n, WiFi, Bluetooth, and ZigBee receiver. The following description of the embodiments of the present invention is provided with reference to WiFi 802.11g/n, Bluetooth, and ZigBee communications protocol or standards. It is understood however that embodiments of the present invention are equally applicable to many other communication protocols.

A backscatter communication system, in accordance with embodiments of the present invention, may use commodity radios by using codeword translation. As is known, any wireless signal on the ISM band is generated using a set of known codewords from a fixed codebook. For example, Bluetooth uses FSK modulation and has two codewords in its codebook: it transmits a tone at one frequency to send a one, and a different frequency to send a zero. Similarly, WiFi and ZigBee also have finite sets of codewords that vary in combinations of phase, amplitude or frequency.

To perform codeword translation, a tag transforms (or translates) the ongoing excitation signal's codeword into another valid codeword in the same codebook during backscattering. This is achieved by modifying one or more of the amplitude, phase, or frequency of the excitation signal. The specific translation depends on the data that the tag seeks to communicate and the type of the excitation signal. Because the codeword in the backscattered signal is a valid codeword from the same codebook as the original excitation signal, a commodity radio may be used to receive the backscattered signal.

FIG. 1 is a simplified view of a backscatter communication system 100, in accordance with one embodiment of the present invention. An ISM band radio, which may be a WiFi, Bluetooth or ZigBee transmitter, transmits data in the form of packets 15 during normal operation to receiver 30, which may be a commodity receiver, such as WiFi, Bluetooth or ZigBee. An internet-of-things (IoT) device 20 (which is also referred to herein as a tag) also receives packets 15, implements codeword translation to embed the information that tag 20 seeks to transmit as described further below, and backscatters the codeword translated packet 25 to receiver 40, which may also be a commodity receiver, such as WiFi, Bluetooth or ZigBee. The backscattered signal (alternatively referred to herein as packet) 25 is frequency shifted to another channel. Accordingly, receiver 30 receives and decodes the originally transmitted packets 15 and receiver 40 receives and decodes the backscattered packets 25. Decoder 50 is configured to compare the packets received by receivers 30 and 40 thereby to retrieve the tag data embedded by tag 20.

Embodiments of the present invention support multiple tags on the same wireless channel by leveraging packet length modulation to transmit necessary information to the tags for coordination. To achieve this, the length of the excitation packet 15 is used to encode 0s and 1s, which can be arranged to form messages to the tags that implement a backscatter MAC protocol. The protocol thus sends control messages to the tags so that the tags can coordinate their transmissions to avoid collisions.

Embodiments of the present invention achieve, among other things, the following objectives. One embodiment decodes backscattered OFDM WiFi signals from 42 m in a line-of-sight (LOS) deployment, and 22 m in a non-line-of-sight (NLOS) deployment. One embodiment achieves a throughput of nearly 60 kbps from a backscattered OFDM WiFi signal when a LOS receiver is 18 m or closer. For farther distances, in one example, an average of 32 kbps (LOS) and 20 kbps (NLOS) is achieved. One embodiment backscatters ZigBee signals from up to, for example, 22 m, achieving 15 kbps. One embodiment backscatters Bluetooth signals up to, for example, 12 m, achieving 55 kbps. A backscatter tag, in accordance with embodiments of the present invention may coexists with WiFi networks independent of the type of excitation signal the tag is backscattering. Furthermore, in one experimental setup, up to 20 backscatter tags were shown to operate effectively while communicating successfully in a MAC scheme and maintaining uplink fairness.

When a tag, in accordance with embodiments of the present invention, backscatters an excitation signal, the tag may modify one or more of the signal's amplitude, phase, or frequency. Such modification is shown below where S(t) represents the excitation signal, T(t) represents the tag signal, and B(t) represents the backscattered signal. The backscattered signal B(t) is the time domain product between the excitation signal S(t) and the tag signal T(t). Therefore, a tag may change its signal T(t) to modify the amplitude, phase, and frequency of the backscattered signal B(t). Signals S(t), T(t), and B(t) may be represented as shown below:

$\begin{matrix} {{{S(t)} = {A_{s}e^{j{({{2\pi\; f_{s}t} + \theta_{s}})}}}}{{T(t)} = {A_{t}e^{j{({{2\overset{.}{\pi}\; f_{t}t} + \theta_{t}})}}}}\begin{matrix} {{B(t)} = {{S(t)}{T(t)}}} \\ {= {A_{s}e^{j{({{2\pi\; f_{s}t} + \theta_{s}})}}A_{t}e^{j{({{2\pi\; f_{t}t} + \theta_{t}})}}}} \\ {= {A_{s}A_{t}e^{j{({{2{\pi{({f_{s} + f_{t}})}}t} + {({\theta_{s} + \theta_{t}})}})}}}} \end{matrix}} & (1) \end{matrix}$

A tag, in accordance with embodiments of the present invention, is configured to modify the amplitude of the backscattered signal by tuning the terminating impedance of the tag antenna. The backscattered signal B(t) strength is defined by:

$\Gamma = \frac{Z_{T} - Z_{A}^{*}}{Z_{A} + Z_{T}}$

In the above expression, Z_(A) represents the tag antenna impedance and Z_(T) represents the impedance across tag antenna terminals A more exact relationship between the backscattered signal strength and Γ may be seen in “Hybrid analog-digital backscatter: A new approach for battery-free sensing in RFID (RFID)”, authored by Vamsi Talla and Joshua R Smith, IEEE International Conference on IEEE, 2013, pp. 74-81.

In a conventional backscatter system, a tag switches between Z_(T) ₁ =Z_(A) and Z_(T) ₂ =0 to encode information. Therefore, two levels of amplitude are seen on a backscattered signal. Instead of switching between two impedances, as is done conventionally for creating the analog backscatter signal, a tag, in accordance with embodiments of the present invention varies across a multitude of impedances to fine tune the amplitude of the backscattered signal.

A tag, in accordance with one embodiment of the present invention, changes the phase of the backscattered signal by delaying the tag signal. In order to introduce an additional phase offset Δθ at the tag, the tag signal is delayed by

$\frac{\Delta\;\theta}{2\pi\; f_{t}}.$ The phase onset Δθ introduced at the tag leads to a phase offset on the backscattered signal. To change the frequency of the backscattered signal, the tag changes the toggling frequency of its RF transistor. Therefore, a tag, in accordance with embodiments of the present invention, is configured to modify the amplitude, phase, and frequency of the backscattered signal, thereby to enable backscatter communication between commodity radios. Codeword Translation

To communicate with a commodity radio, a backscatter tag, in accordance with one embodiment of the present invention, performs codeword translation, as described further

A codeword C_(i) is defined herein as a signal symbol on the physical layer that represents specific data transmitted. For example, Bluetooth uses binary FSK modulation to embed information. Therefore, it only uses two codewords, C₁=e^(j2πf) ¹ ^(t) and C₂=e^(j2πf) ² ^(t) to represent data one and data zero respectively.

A codebook B is the set of valid codewords used by a radio. The codebook associated with Bluetooth is B={C₁, C₂} because only two codewords are used in the Bluetooth standard. Similarly, WiFi 802.11g/n standard uses a codebook B={C₁, C₂ . . . C_(n)}, where C_(i), wherein i is an index ranging from 1 to n, and is an OFDM symbol. As is well known, the WiFi, ZigBee and Bluetooth use different sets of codewords and codebooks.

Different codewords in the same codebook are related to each other by a shift in phase, amplitude, frequency or a combination thereof. For example, the codeword C₁ used by the Bluetooth standard only differs from C₂ in the Bluetooth standard in the frequency domain, with a frequency difference of f₂−f₁.

Codeword translation is the act of transformation of a valid codeword C_(i) to another valid codeword C_(j) where both codewords belong to the same codebook, meaning C_(i) ∈ B and C_(j) ∈ B. A backscatter tag, in accordance with embodiments of the present invention, performs such translation in compliance with WiFi, ZigBee, and Bluetooth standards, while consuming a relatively small amount of power. Because the transformed/translated codeword remains a valid codeword in the same codebook, a commodity WiFi, ZigBee, or Bluetooth radio may be used to decode the backscatter signal. The tag data is encoded by the specific codeword translations, as described further below.

An example of codeword translation performed by a tag, in accordance with embodiments of the present invention is shown in expression (2) below, where the codeword of the excitation signal is C_(i). To encode a one, the tag translates the codeword C_(i) to C_(j) before transmission. To encode a zero, the tag leaves the codeword untranslated, therefore, the backscatter signal has the same codeword as the excitation signal.

$\begin{matrix} {{{backscatter}\mspace{14mu}{codeword}} = \left\{ \begin{matrix} C_{j} & {{Tag}\mspace{14mu}{data}\mspace{14mu}{one}} \\ C_{i} & {{Tag}\mspace{14mu}{data}\mspace{14mu}{zero}} \end{matrix} \right.} & (2) \end{matrix}$

By using codeword translation, the tag decodes the backscatter signal using commodity WiFi, ZigBee, or Bluetooth radios to extract the tag data. Table I below shows the logic table for decoding a backscatter signal.

TABLE I decoded codeword excitation signal codeword tag bit C₂ C₁ 1 C₁ C₂ 1 C₁ C₁ 0 C₂ C₂ 0

As is seen from Table I, the tag bits are the XOR function of the backscattered codeword and the original codeword. Therefore, the tag data may be extracted by computing the XOR of the original excitation bitstream and the backscatter bitstream.

As was described above, a tag performs codeword translation by modifying the amplitude, phase, or frequency of the excitation signal. Such modification transforms the excitation codeword from C_(i) to C_(j) in the backscattered signal. A tag performing codeword translation is frequency agnostic, and thus applies the same modification on signals across all frequencies. This does not pose any problems for standards that use a single carrier wave, such as Bluetooth, ZigBee, and 802.11b standards. However, because an OFDM signal associated with the 802.11n standard uses multiple subcarriers, the above codeword translation can cause problems. When a tag changes the amplitude of a signal on subcarrier it will introduce the same amplitude modification on another subcarrier m. However, the modified signal on subcarrier m may not be a valid codeword.

An example of this is shown in FIG. 2 where the data modulated on subcarrier i is 1000 while the data modulated on subcarrier m is 0101. When the tag transforms the signal on subcarrier i from 1000 to 1101 by reducing the signal amplitude, the tag applies the same operation on subcarrier m, reducing the strength of the signal that represents 0101. As a result, the tag creates an invalid codeword on subcarrier m. Therefore, when a tag does codeword translation, it looks for a signal characteristics, such as amplitude, phase, or frequency, to perform signal modification/translation such that the modified signal remains a valid codeword.

Backscatter of OFDM Symbols

Equation 3 below shows an OFDM modulated signal where {X_(k)} are the data symbols modulated on subcarriers, N is the number of sub-carriers, and T is the OFDM symbol time. For 802.11g/n standard, an OFDM symbol lasts for 4 μs and contains 64 subcarriers. The data symbols {X_(K)} are generated using BPSK, QPSK, 16-QAM, or 64-QAM modulation depending on the WiFi standard bit rate.

$\begin{matrix} {{S(t)} = {\sum\limits_{k = 0}^{N - 1}\;{X_{k}e^{\frac{2\pi\;{kt}}{T}}}}} & (3) \end{matrix}$

When backscattering an OFDM symbol, a tag, in accordance with one embodiment of the present invention, does not modify the amplitude or frequency of the excitation OFDM signal because such modification creates an invalid codeword in the backscattered signal. Therefore, the tag modifies only the phase of the backscattered signal. A binary example is shown in equation 4 below. The tag introduces a phase offset Δθ to transmit a data one. It introduces no offset to transmit a data zero. The value of Δθ depends on the tag bit rate. For example, if the tag transmits a lower data rate, it uses the binary scheme where Δθ is 180°. If the tag decides to transmit at higher data rate, it may choose Δθ as 90° and use equation 5, shown below, to encode its information.

$\begin{matrix} {{B(t)} = \left\{ \begin{matrix} {{S(t)}e^{j\; 0}} & {{Tag}\mspace{14mu}{data}\mspace{14mu} 0} \\ {{S(t)}e^{j\;\Delta\;\theta}} & {{Tag}\mspace{14mu}{data}\mspace{20mu} 1} \end{matrix} \right.} & (4) \\ {{B(t)} = \left\{ \begin{matrix} {{S(t)}e^{j\; 0}} & {{Tag}\mspace{14mu}{data}\mspace{14mu} 00} \\ {{S(t)}e^{j\;\Delta\;\theta}} & {{Tag}\mspace{14mu}{data}\mspace{14mu} 01} \\ {{S(t)}e^{j\; 2\Delta\;\theta}} & {{Tag}\mspace{14mu}{data}\mspace{14mu} 10} \\ {{S(t)}e^{j\; 3\Delta\;\theta}} & {{Tag}\mspace{14mu}{data}\mspace{14mu} 11} \end{matrix} \right.} & (5) \end{matrix}$ Backscatter with ZigBee

A ZigBee radio uses Offset QPSK (OQPSK) modulation. Similar to QPSK modulation, the data is encoded in the phase of the transmitted signal. Therefore, a tag, in accordance with embodiments of the present invention, embeds data in an OQPSK signal by modifying the phase during reflection. When the tag transmits a data one, it introduces a Δθ phase offset on the reflected signal. When the tag transmits a data zero, it does not change the phase. The formula for embedding tag bits in ZigBee is the same for an 802.11g/n WiFi standards shown in equations 4 and equation 5 above.

Backscatter with Bluetooth.

A Bluetooth radio modulates information by changing the carrier signal frequency between two frequencies f₁ and f₀ depending on the codeword transmitted. To transmit a data one, the radio sends a sine wave with frequency f₁. To transmit a data zero, the radio sends a sine wave with frequency f₀. A tag, in accordance with the present invention, uses the formula shown in expression (6) below to embed its information. When transmitting data one, the tag generates an additional frequency offset Δf in the backscattered signal by toggling its RF transistor at frequency Δf. When transmitting data zero, the tag does not generate the additional frequency offset. If we select Δf carefully, we can ensure that B(t) is still a valid Bluetooth signal and can be decoded by a commercial Bluetooth radio

$\begin{matrix} {{B(t)} = {{{S(t)}{T(t)}} = \left\{ \begin{matrix} {{S(t)}e^{j{({2{\pi\Delta}\; f\; t})}}} & {{tag}\mspace{14mu}{data}\mspace{14mu}{one}} \\ {S(t)} & {{tag}\mspace{14mu}{data}\mspace{14mu}{zero}} \end{matrix} \right.}} & (6) \end{matrix}$

One possible option to ensure that the B(t) remains a valid Bluetooth signal is to select Δf to be defined by |f₁−f₀|. Assume that the Bluetooth radio transmits a data one with frequency f₁. For the tag to transmit data one, it shift the signal by Δf so that the backscattered codeword ise^(j(2πf) ⁰ ^(t+θ) ^(S) ⁾. This is a valid Bluetooth FSK code word because it is a sine wave with frequency f₀. However, a commercial Bluetooth radio will decode it as a zero rather than a one. Conversely, to encode a data zero the tag does not frequency-shift the Bluetooth signal. The case is symmetric when the Bluetooth radio transmits data zero with frequency f₀ instead. Therefore, to transmit a data one, a tag, in accordance with one embodiment of the present invention, transforms a Bluetooth codeword with frequency f₁ to a backscattered codeword with frequency f₀, and transforms a Bluetooth codeword with frequency f₀ to a backscattered codeword with frequency f₁. To transmit a data zero, the tag generates a backscattered codeword with the same frequency as the original Bluetooth codeword. Therefore, by selecting Δf as described above, a tag, in accordance with one embodiment of the present invention, generates a backscattered signal that is a valid Bluetooth signal while embedding the data it seeks to transmit.

Avoiding Interference from Active Radios

If a tag transmits a backscatter signal to a receiver, the receiver may see severe interference from the excitation signal because both the backscattered signal and the excitation signal share the same channel. To avoid such interference, a tag, in accordance with embodiments of the present invention, shifts the frequency of the backscatter signal to ensure that it occupies a frequency channel different from the one occupied by the excitation signal. Such frequency shifting techniques are described, for example, in a paper entitled “Enabling practical backscatter communication for on-body sensors”, authored by Pengyu Zhang, Mohammad Rostami, Pan Hu, and Deepak Ganesan, Proceedings of the 2016 conference on ACM SIGCOMM 2016, pp. 370-383, or in a paper entitled “Inter-Technology Backscatter: Towards Internet Connectivity for Implanted Devices”, authored by Vikram Iyer, Vamsi Talla, Bryce Kellogg, Shyamnath Gollakota, and Joshua Smith, Proceedings of the 2016 conference on ACM SIGCOMM 2016, pp. 356-369.

Such frequency shifting may be achieved, for example, by toggling the RF transistor at the desired frequency offset. For example, if to shift the backscattered signal 20 MHz away from the excitation signal, the RF transistor is toggled at 20 MHz. In one example, when backscattering a WiFi signal, the tag shifts the frequency such that the backscattered signal is tuned to, for example, channel 13, which is the least used channel in the 2.4 GHz ISM band. Such channel allocation reduces interference to and from other active radios. When backscattering Bluetooth or ZigBee, the tag shifts the frequency of the backscatter signals so that they are tuned to channels close to 2.48 GHz because these channels experience less interference from the WiFi signal.

MAC Protocol

To facilitate effective sharing of the wireless medium between multiple tags, a media access (MAC) scheme is developed in accordance with embodiments of the present invention. The MAC protocol serves two purposes, namely it informs the tag what signals to backscatter with, and further it provides support for multiple tags, as described further below.

Coordinating Tags

Determining when to backscatter is important. If the incorrect signal is backscattered, data cannot be recovered. The tags need a way to distinguish when to start backscattering signals. To ensure that the tag starts to backscatter at the appropriate time, the transmitter (e.g., transmitter 10 in FIG. 1) sends a preamble containing a predetermined sequence of 0s and 1s, described further below. The tag maintains a circular buffer of received bits. If the beginning of the buffer matches the preamble, the tag knows that the buffer contains backscatter data initiated by a command from the transmitter and not random packets.

Communicating with Multiple Tags

Because a tag does not have sufficient power to perform carrier sensing, a random access scheme based on Framed Slotted Aloha protocol is used. In accordance with this protocol, the transmitter acts as a central coordinator, in the same manner as described in the publication “An empirical study of UHF RFID performance” by Michael Buettner and David Wetherall, Proceedings of the 14th ACM international conference on Mobile computing and networking, 2008, pp. 223-234. Communication is carried out in rounds with a fixed number of slots per round. In each round, the tags choose a random slot to transmit. If two tags choose the same slot, collision occurs and data is not successfully transmitted. At the end of a round, the transmitter processes data from the tags and adjusts the number of slots before proceeding to the next round.

Compared to a stochastically allocated time-division scheme, random access allows the number of tags to grow and shrink without a specific association process. The number of slots is inferred by the receiver from the number of packets it receives, as well as the number of possible collisions. The receiver passes this information to the transmitter (e.g., transmitter 10 in FIG. 1). If the transmitter sees many collisions, it increases the number of slots. If the number of collisions fall below a predefined number, the transmitter decreases the number of slots. To avoid collisions from other users on the same channel, the transmitter, such as a transmitter 10 shown in FIG. 1, uses carrier sensing before sending messages to the tags. Each round can have an arbitrary amount of delay before the next. This ensures that the backscatter system does not hog the channel. The use of rounds allows for fairness between the backscatter system and other users of the channel. The use of slots within the backscatter system allows for fairness between tags.

FIG. 3 shows the durations of 30 million packets on channel 6 collected in a lecture hall. In the bimodal distribution seen in FIG. 3, nearly 78% of packets last less than 500 μs and nearly 18% of the packets last 1500 μs-2700 μs. With a pulse-width error bound of 25 μs, the probability of an ambient packet having the same length as packets in accordance with one embodiment of the present invention is about 0.03%.

Transmitting Coordination Messages to Tags

In accordance with one aspect of the present invention, the communication from the transmitter to the tags is performed using a technique that consumes relatively low power and dispenses with the need for the tag to decode packets. To achieve this, in one embodiment, an envelope detector is used to enable communication between the transmitter and the tag. Low-power envelope detectors typically consumes less than 1 μW. Such an envelope detector is configured to measure parameters that can be easily measured and modulated at the transmitter using, for example, commodity hardware, as described further below.

Packet Length Modulation

In accordance with one embodiment of the present invention, packet length modulation (PLM) is used to establish communication from the transmitter to the tags. Packet duration is relatively easy for the transmitter to control, works well at a range of distances, and is robust in the presence of ambient network traffic. In the PLM scheme used in accordance with one embodiment of the present invention, a 0 bit is represented by packets of duration L₀ and a 1 bit is represented by packet of duration L₁. To control the length of the packet, the transmitter sends packets of pre-defined durations L₀ and L₁. The tag uses an envelope detector to identify the presence and duration of a packet. If a packet duration equals L₀ or L₁ (within a predefined error range) a bit is recorded to a data buffer. If a packet has a duration different than L₀ or L₁ (taking into account the predefined range) the packet is treated as noise and discarded, thereby enabling the bits to be received successfully in the presence of other transmissions.

In one embodiment, a backscatter tag, in accordance with the present invention, operating using a WiFi 802.11 g/n standard operates at approximately 500 bps, which is sufficient for operating the MAC layer.

To send the scheduling messages, the transmitter may generate dummy packets. Alternatively, the transmitter may buffer existing traffic before sending it to the network interface card (NIC), and then reorder or repacketize to form the sequence of L₀ and L₁s. Therefore, as long as the network is busy, the backscatter messages imposes negligible overhead on the rest of the channel.

FIG. 4 shows the rate of decoding success as a function of distance. The experiment associated with FIG. 4 was performed in a long hallway inside an office building. For a reference voltage of 1.8v, the system is seen as being able to successfully decode the scheduling messages with over 70% accuracy when the tag is less than 4 m away from the transmitter. The system is seen as successfully decoding the preamble with about 50% a distance of 50 m. Due to increased SNR, higher accuracy is possible at close proximity by increasing the reference voltage in the comparator. A prototype system was built using off-the-shelf commodity 802.11g/n WiFi, ZigBee, and Bluetooth transceivers and a custom made backscatter tag, as described further below.

FIG. 5 is a high-level simplified block diagram of a backscatter tag 100, in accordance with one embodiment of the present invention. Backscatter tag 100 is shown as including, in part, a multiplexer 110, a codeword translator 105, a phase modulator 115, a frequency modulator 120, a frequency shifter 125, and an antenna 130. Depending on the type of the data received and the communication protocol used, codeword translator uses the output of either phase modulator 115, or frequency modulator 120. Phase modulator 115 modulates the phase of the received packets and the frequency modulator modulates the frequency of the received packets, as described in detail above. The output of multiplexer 110 is frequency shifted by frequency shifter 125 before being transmitted by antenna 130.

Hardware Platform

802.11 g/n Transceiver

An 802.11g/n receiver disposed in a MacBook Pro laptop with a Broadcom BCM43xx WiFi card that supports 802.11a/b/g/n/ac was used. The WiFi card was placed into a monitor mode to report packets that had incorrect checksums. After receiving the packets, tcpdump (a well-known software) was used to parse the packets and extract the tag bits.

Also an Intel 5300 WiFi card on an Intel NUC was used as the standard 802.11g/n OFDM transmitter, transmitting at 15 dBm. The firmware used was the one described in “Tool release: gathering 802.11 n traces with channel state information provided”, authored by Daniel Halperin, Wenjun Hu, Anmol Sheth, and David Wetherall, ACM SIGCOMM Computer Communication Review 41, 1 (2011), 53-53.

ZigBee Transceiver:

A TI CC2650 radio (http://www.ti.com/product/CC2650) was used as the ZigBee transceiver whose the transmission power was set to 5 dBm, which is the maximum power allowed by this radio. The CC2650 radio development board CC2650EM-7ID supports two types of antennas: a PCB on-board antenna and an antenna with an SMA interface. In the experiment, the VERT2450 antenna was used because it has a wider beam. It was mounted on the SMA interface, as described in “Ettus Research. [n.d.]. VERT2450 Antenna. https://www.ettus.com/product/details/VERT2450.

Bluetooth Transceiver:

A TI CC2541 radio (http://www.ti.com/product/CC2541) was used as the Bluetooth transceiver. This radio transmits at 1 Mbps and 0 dBm using FSK modulation with a frequency deviation of 250 kHz and a bandwidth of 1 MHz. The modulation index used is 0.5±0.01.

Tag:

The tag used has two VERT2450 antennas: one for reception and one for transmission https://www.ettus.com/product/details/VERT2450. The reception antenna is coupled to an LT5534 envelope detector, which measures when an incoming signal starts. A 0.35 us delay was measured between the starting point of an excitation signal and the indicator signal from the envelope detector. In other words, 0.35 μs after the arrival of the excitation signal, the envelope detector notifies the processor that the excitation signal has begun. In the evaluations, the performance does not degrade when experiencing a 0.35 μs delay.

The other antenna is controlled by an ADG902 RF switch, which decides when and how to backscatter the excitation signal. The codeword translation module is implemented in a low-power FPGA AGLN250. A power management module was used on the tag which provides 1.5V and 3.3V to the rest of the system. The source code of the tag platform is available at https://github.com/pengyuzhang/FreeRider.

Implementation Challenges

Each radio comes with its own physical layer stack with a specific set of channel codes, interleaving techniques and scrambling algorithms, all of which can interfere with codeword translation and render it ineffective. A description of how to enable codeword translation in the presence of these challenges is provided below.

Challenges in Backscattering OFDM WiFi Signals

FIG. 6 shows various block diagrams of an 802.11 g/n transmission and reception blocks. There are three factors that could cause difficulty when decoding a backscattered WiFi signal, namely the scrambler, the convolutional channel encoder, and interleaving. The scrambler is a data whitening engine where it takes the input data and XORs it with a pseudorandom sequence. A scrambler ensures that the data transmitted is not all zeros nor all ones, which causes a bad peak-to-average ratio. The channel encoder uses convolutional encoding to improve its robustness over wireless transmission. The interleaving engine re-orders the transmitted bits sequence to ensure that even a bursty error on wireless channel does not cause a burst of continuous errors on the received data. These three modules are described below because they are placed before the modulator in an 802.11g/n transmitter. An explanation as to why such placement could cause unsuccessful backscatter decoding is also described below.

For any input sequence b₀, b₁ . . . b_(n), the transmitted signal S(t) can be formulated as S(t)=f(b₀, b₁, . . . , b_(t)) where f( ) represents the operations introduced by the scrambler, channel encoder, interleaver, and modulator. Since the corresponding demodulator, de interleaver, channel decoder, and descrambler in the receiver provide the reverse operations f⁻¹( ), the receiver is able to decode and output the transmitted sequence.

However, when a tag is present and produces a signal g(b₀, b₁, . . . , b_(t)) using tag bits t₀, t₁, . . . , t_(n), the backscattered signal B(t) becomes the time-domain product between the tag signal and the excitation signal. To help understand this further, the binary case shown in equation 7 below is explained. The signal does not look like it is generated by XORing the excitation signal bits with the tag bits and passed through f( ). Therefore, decoding the tag bits becomes hard. B(t)=S(t)T(t) =f(b ₀ ,b ₁ , . . . ,b _(n))×g(t ₀ ,t ₁ , . . . ,t _(n)) !=f(b ₀ ⊕t ₀ ,b ₁ ⊕t ₁ , . . . ,b _(n) ⊕t _(n))  (7)

A possible solution to this problem is redundancy, i.e., map one tag bit to multiple 802.11g/n bits. Instead of directly transmitting t₀, t₁, . . . , t_(n) the tag transmits a sequence where a tag repeats each bit multiple times before switching to the next one. The following is a description of the reason why such redundancy helps solve the problem.

The interleaving module is configured to interleave the data assigned to each subcarrier. Interleaving is done per OFDM symbol. In other words, the interleaving module does not interleave data belonging to two OFDM symbols. Therefore, as long as the tag bit duration is longer than an OFDM symbol, the interleaving module will not cause problems.

Both the scramble and channel encoder modules generate and maintain a deterministic structure of the data delivered to the modulator. The scrambler uses the structure shown in FIG. 7 to do data whitening. Even when the input is all zeros, the actual data transmitted is a non-zero sequence. Such data whitening reduces the peak-to-average power ratio in the RF front end. The mathematical expression of the scrambler is shown in equation 8: C[k]=b[k]⊕b[k−3]⊕b[k−7]  (8)

The channel encoder uses equation 9, shown below, to encode the data at 6 Mbps where b(k) is the input bit C₁(k) and C₂(k) are the codewords generated using a ½ coding rate. C ₁[k]=b[k]⊕b[k−2]⊕b[k−3]⊕b[k−5]⊕b[k−6] C ₂[k]=b[k]⊕b[k−1]⊕b[k−2]⊕b[k−3]⊕b[k−6]  (9)

For other bit rates, the channel encoder is different. The data injected by the tag may corrupt the structures created by the two modules and make backscatter decoding difficult. To overcome these challenges, the two modules were simulated using Matlab and it was found that as long as a tag injects one bit tag data on four OFDM symbols (96 WiFi bits in 6 Mbps data rate), an error bit rate of nearly 1e⁻³ may be ontainted. This is because there is a one-to-one mapping between the input sequence b(k) and the output of the two modules C(k) or {C₁[k], C₂[k]}.

Equation 8 and equation 9 show that the sequence of {b[k]⊕1, b[k−1]⊕1, . . . b[k−7]⊕1} can generate C[k]⊕1 and {C₁[k]⊕1, C₂[k]⊕1}. Therefore, when the tag does codeword translation and converts C(k) and {C₁[k], C₂[k]} to C[k]⊕1 and {C₁[k]⊕1, C₂[k]⊕1}, the corresponding modules at the receiver should output {b[k]⊕1, b[k−1]⊕1, . . . , b[k−7]⊕1}. This result has been proven using empirical Matlab simulation and real system implementation with a MacBook Pro laptop as the backscatter decoder.

The last factor that may impact backscatter decoding is the pilot tone. Pilot tones in an OFDM symbol are used for correcting the phase error. Such phase error correction could remove the additional phase offset introduced by a tag, and render incorrectly decoded tag data. However, there are a number of WiFi chips, such as Broadcom's BCM43xx, that do not use pilot tones for phase error correction and are this able to correctly decode the backscattered tag data.

Challenges in Backscattering ZigBee

ZigBee uses OQPSK modulation where there is a constant time-domain offset (half a bit) between the in-phase signal and the quadrature signal. Such offset is introduced for reducing the signal Peak-to-Average Power Ratio (PAPR) by avoiding the 18 phase transition between neighboring bits. If the tag introduces a 180° phase transition between neighboring bits in the backscattered ZigBee, it may corrupt the OQPSK signal structure and cause trouble decoding.

One solution to this problem is embedding one tag bit to each multiple (N) OQPSK symbols. When a tag transmits data one, instead of introducing the 180° phase offset on a sine OQPSK symbol, the tag introduces the same 180° additional phase offset on N OQPSK symbols. The first tag-modified OQPSK symbol might be incorrectly decoded by a commercial ZigBee decoder because of the potential OQPSK signal structure violation described above. However, the following N−1 tag-modified OQPSK symbols can be correctly decoded because the structure of OQPSK signals is maintained. Therefore, as long as a relatively large N is selected, the data information may be embedded in ZigBee traffic. In one example, N was selected to have a value of 8.

Challenges in Backscattering Bluetooth

There are two challenges to overcome in decoding a backscattered Bluetooth signal, namely modulation index i, and channel bandwidth w. Modulation index i is defined as

$\frac{f_{1} - f_{0}}{w}$ and represents the ratio between the frequency deviation of an FSK signal and the bandwidth it occupies. A commercial Bluetooth radio usually uses a modulation index 0.5. When a tag, in accordance with embodiments of the present invention, toggles its RF transistor at Δf, while generating the desired backscattered signal, the tag also generates an undesired signal on the other side of the spectrum as shown in FIG. 8. This undesired signal is generated because the backscattered signal is the time-domain product between the Bluetooth signal and the tag signal. Therefore a double-sideband backscatter signal is generated.

In accordance with one embodiment of the present invention, the undesired backscattered signal is eliminated by taking advantage of the fact that a Bluetooth radio treats signals outside of a channel as interference and is able to eliminate them. Therefore, the selection of Δf needs to satisfy the following two conditions to ensure that the undesired signal remains outside of the backscatter channel and is thus eliminated:

$\begin{matrix} \left\{ \begin{matrix} {{f_{1} + {\Delta\; f}} > {f_{1} + {\left( {1 - i} \right)\frac{w}{2}}}} & {{FSK}\mspace{14mu}{radio}\mspace{14mu}{data}\mspace{14mu}{one}} \\ {{f_{0} - {\Delta\; f}} < {f_{0} - {\left( {1 - i} \right)\frac{w}{2}}}} & {{FSK}\mspace{14mu}{radio}\mspace{14mu}{data}\mspace{14mu}{zero}} \end{matrix} \right. & (10) \end{matrix}$ Low-Power Tag Design

To achieve low power consumption, a tag, in accordance with embodiments of the present invention uses a ring oscillator to generate the square wave signals needed for achieving the frequency shifting. One such design is described in the article “Enabling practical backscatter communication for on-body sensors”, authored by Pengyu Zhang, Mohammad Rostami, Pan Hu, and Deepak Ganesan, proceedings of the conference on ACM SIGCOMM 2016, pp. 370-383. In one specific prototype formed using a 65 nm technology node, the overall power consumption of the tag is nearly 30 μW depending on the excitation signal. Most of the power (e.g., 19 μW) is consumed by generating the 20 MHz clock needed for frequency shifting. It was determined that 12 μW was used for operating the RF switch and 1-3 μW was used for running the control logic which determines the type of the codeword translator to run.

Experimental Setup

FIGS. 9A and 9B respectively show experiments for testing a tag's performance when the tag is deployed in a line-of-sight (LOS) and non-line-of sight (NLOS) setup. The tag was positioned 1 m away from the transmitter (802.11g/n WiFi, ZigBee, or Bluetooth). No hardware modifications were performed on the commodity radio transmitters. The receiver was then moved away from the tag as the tag's throughput, bit error rate (BER), and received signal strength indicator (RSSI) were measured. In the LOS experiments, all devices are placed in a hallway. In the NLOS experiments, the transmitter and the tag are deployed in a room while the receiver is deployed in a hallway. In the NLOS deployment, the backscattered signal passes through multiple walls.

Tag's Backscatter Performance with 802.11g/n WiFi Deployed in LOS

FIG. 10A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a function of distance between the tag and the receiver in a LOS deployment. The 802.11g/n WiFi transmitter sends its OFDM signal at 11 dBm. It is seen that the receiver remains able to decode the backscattered signal at a distance of 42 m. This distance is 1.4 times longer than the maximum distance reported in “Passive WiFi: bringing low power to Wi-Fi transmissions”, authored by Bryce Kellogg, Vamsi Talla, Shyamnath Gollakota, and Joshua R Smith, 2016, 13th USENIX Symposium on Networked Systems Design and Implementation (NSDI 16) 151-164; and 8.4 times longer than the maximum distance achieved by FS-Backscatter, as reported in “Enabling practical backscatter communication for on-body sensors”, authored by Pengyu Zhang, Mohammad Rostami, Pan Hu, and Deepak Ganesan, Proceedings of the 2016 conference on ACM SIGCOMM. Such long communication distance is sufficient for many Internet-of-Things applications.

A tag, in accordance with one embodiment of the present invention, achieves nearly 60 kbps data rate when the receiver is less than 18 m away from the tag. When the receiver moves farther within nearly 26 m-36 m from the tag, the throughput decreases to nearly 15 kbps. This is a relatively lower data rate because OFDM symbols are longer in duration than DSSS symbols. It is seen that the bit error rate remains low even at longer distances as shown in FIG. 10B despite the fact that RSSI does degrade across distance as shown in FIG. 10C. For example, a bit error rate of 1e⁻³ is achieved when the receiver is positioned 40 m away from the tag. Accordingly, at longer distances, if a backscattered packet reaches the receiver, then it is very likely that the tag will be able to extract the bits with low BER. If the header itself is not decoded, then packet loss increases and throughput is degraded.

Tag's Backscatter Performance with 802.11g/n WiFi Deployed in NLOS

In the NLOS experiment, the 802.11g/n transmitter and the tag were placed in a room while the receiver was moved away in a hallway. FIG. 11A shows the throughput of the system in this setup. The receiver is able to receive the backscattered packets when it is 22 m away from the tag Similar to the LOS deployment, a data rate of nearly 60 kbps is achieved when the receiver is less than 14 m away from the tag. At longer distances, the backscatter throughput degrades to nearly 20 kbps. FIG. 11B shows the BER of the system in the NLOS deployment. Similar to the LOS deployment, a low BER is achieved across various distances. However, backscatter communication appears to stop at 22 m even though an RSSI of −84 dBm is achieved at 22 m as shown in FIG. 11C. It is seen that when the receiver is more than 22 m away from the tag, the backscattered signal needs to pass one more wall before reaching the receiver as shown in FIG. 9B. As a result, the signal becomes too weak and the packet header is not detected.

Backscatter with ZigBee

FIG. 12A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a ZigBee receiver moves away from the tag. The receiver receives backscattered packets from nearly 22 m away. FIG. 12B shows the BER (bit error rate) of a tag, in accordance with one embodiment of the present invention, as a ZigBee receiver moves away from the tag. FIG. 12C shows that the received signal strength degrades to −97 dBm at 22 m, which is close to the noise floor of the ZigBee radio. Therefore, receiving the backscattered packets at longer distances becomes challenging. A backscatter data rate of nearly 14 kbps is achieved when the receiver is less than 12 m away from the tag. At farther distances, the throughput degradation is not severe. A data rate of 12 kbps is achieved at a distance of 20 m. The bit error rate achieved is nearly 5e⁻² across all distances, higher than the case when the excitation signal is 802.11g/n.

Backscatter with Bluetooth

FIG. 13A shows the throughput of a tag, in accordance with one embodiment of the present invention, as a Bluetooth receiver moves away from the tag. It is seen that the receiver decodes backscatter packets up to 12 m. FIG. 13B shows the BER of a tag, in accordance with one embodiment of the present invention, as a Bluetooth receiver moves away from the tag. FIG. 13C shows that the backscatter signal has a strength of nearly −100 dBm at 12 m, close to the noise floor. Therefore, decoding the backscattered packets at farther distance becomes challenging. When the receiver is less than 10 m away from the tag, a data rate of nearly 50 kbps is achieved. Throughput degrades to 19 kbps at 12 m and the BER increases to 0.23.

Impact of Distance Between Transmitter and Tag

To measure this effect, the distance between the tag and the transmitter was varied up to a point where backscatter communication could be sustained. FIG. 14 shows the result of this experiment. When backscattering 802.11g/n signal, at a transmitter-to-tag distance of 4 m, the maximum receiver-to-tag distance is measured as 8 m. This is less than the 42 m achievable when the transmitter-to-tag distance is 1 m. Decreasing the receiver-to-tag communication distance yields a slight increase in the achievable transmitter-to-tag distance. The operational regime of the tag system is shown in the area 210 of FIG. 14.

When a ZigBee or Bluetooth radio is used, both the transmitter-to-tag distance and the receiver-to-tag distance become shorter. The maximum transmitter-to-tag distance is 2 m and 1.5 m for ZigBee and Bluetooth radios respectively, and the corresponding operational regime of tag system is marked with 220 and 230 respectively. Both regimes are smaller than when an 802.11g/n signal is used primarily because the transmission power of the ZigBee and Bluetooth radios is smaller (5 dBm and 0 dBm vs 15 dBm).

Co-Existence with WiFi Networks

To determine if a tag, in accordance with embodiments of the present invention, can co-exist with existing WiFi networks, WiFi traffic was generated in which a laptop transfers files via WiFi on channel 6 (2.437 GHz). Then, a backscatter was run on nearly 2.472-2.48 GHz (the exact frequency depends on the type of the excitation signal). A measurement was made to determine how the WiFi traffic and backscatter impact each other when the backscatter channel does not conflict with the WiFi channel.

Does Backscatter Impact WiFi

FIG. 15 shows the WiFi throughput when the backscatter tag is either present or absent. When backscatter is absent, WiFi is able to transmit at nearly 37.4 Mbps median data rate. Then, a backscatter tag is placed 1 m away from the WiFi receiver and the WiFi throughput is measured. The tag runs three codeword translators sequentially, one for backscattering 802.11g/n WiFi, one for ZigBee, and one for Bluetooth. The median WiFi throughput measured is 37 Mbps, 37.9 Mbps, and 36.8 Mbps respectively, close to the WiFi throughput when the backscatter tag is not present. Therefore, a backscatter tag does not cause interference on an existing WiFi traffic.

Does WiFi Impact Backscatter

To determine whether or not concurrent WiFi traffic impacts backscatter decoding, the following experiment was performed. When a tag backscatters its data in a channel that is occupied by WiFi traffic, backscatter throughput degrades to zero because the WiFi traffic is usually approximately 30 dB higher than the backscattered signal. Therefore, backscatter suffers. Therefore, an experiment is performed under a condition where an existing WiFi traffic does not share the same channel as backscatter, so as to understand how backscatter performs in the presence of WiFi traffic on adjacent channels.

FIG. 16(A) shows the backscatter throughput when an 802.11g/n WiFi is used as the excitation signal with the tag backscattering on channel 13 (2.472 GHz), and the WiFi traffic running on channel 6 (2.437 GHz). When the WiFi traffic is absent, 61.8 kbps median backscatter throughput is achieved. When the WiFi traffic is present, the backscatter throughput remains at 61.8 kbps. However, it is seen that the backscatter is able to reach 68 kbps for 20% of the time when the WiFi traffic is absent, and degrades to 35 kbps for 10% of the time when the WiFi traffic is present. Therefore, the presence of WiFi traffic impacts the backscatter throughput. To minimize this impact, technique similar to the one described in “Hitchhike: Practical backscatter using commodity WiFi”, authored by Pengyu Zhang, Dinesh Bharadia, Kiran Joshi, and Sachin Katti, 2016 ACM SENSYS, may be used together with RTS-CTS to reserve the channel for backscatter.

FIGS. 16(B) and 16(c) show the backscatter throughput when a tag backscatters a ZigBee signal and a Bluetooth signal respectively. In both experiments, the tag backscatters on channel 2.48 GHz. It is seen that the backscatter throughput difference between WiFi traffic being present or not is only approximately between 1-2 kbps. Therefore, an existing WiFi traffic does not impact the backscatter performance when a ZigBee or Bluetooth radio is leveraged. One reason is that both such radios are narrowband, and therefore, have better performance in filtering out-of-band interference.

Evaluating MAC Layer Performance

The performance of the system when communicating with multiple tags is also studied. FIG. 17(a) shows the aggregated throughput when respectively 4, 8, 12, 16, and 20 tags are positioned in the path of the transmitter. The aggregate throughput is lower than that of a single tag for two reasons: control overhead and collisions. As the number of tags increases, the aggregated throughput increases. This is due to the relative ratio of control overhead decreasing when more transmission slots are used. If the simulation is extended beyond the 20 tags, the throughput asymptotically approaches to at about 18 kbps. If there are no collisions (i.e. a TDM scheme), the simulation throughput asymptotically approaches to about 40 kbps.

Framed Slotted Aloha is well-suited to applications that have low data needs and where the number of active tags can increase or decrease without warning, such as inventory tracking. More data-intensive applications would benefit from a time division scheme, which would be possible to implement in a tag, in accordance with embodiments of the present invention. The analysis above was limited to a single MAC layer design.

FIG. 17(b) shows the Jain's fairness index, as described in “Throughput fairness index: An explanation”, Technical Report, Department of CIS, The Ohio State University authored by Raj Jain, Arjan Durresi, and Gojko Babic, 1999, when 4, 8, 12, 16, and 20 tags are present. When the number of tags increases, the fairness index stays about the same because the scheduler dynamically allocates a larger number of slots when more tags are present. The averaged fairness index is 0.85 when 20 tags are present, suggesting that most of tags still obtain similar opportunities for data transmission.

The above descriptions of embodiments of the present invention are illustrative and not limitative. For example, the various embodiments of the present inventions are not limited by the communication protocol, 802.11 g/n, Bluetooth, ZigBee or otherwise, used for signal transmission. Other modifications and variations will be apparent to those skilled in the art and are intended to fall within the scope of the appended claims. 

What is claimed is:
 1. A backscatter tag communication device, comprising: a receiver configured to receive a first packet comprising a first plurality of codewords, the first packet conforming to a communication protocol defining a second plurality of codewords inclusive of the first plurality of codewords, wherein the second plurality of codewords comprises two or more codewords, wherein the first packet is characterized by a first frequency, and wherein each codeword comprises a plurality of bits; a codeword translator configured to generate a second packet by, for each respective codeword of the first plurality of codewords and based on a data that the backscatter tag communication device is invoked to transmit: selecting a target codeword of the second plurality of codewords; and translating the respective codeword to the target codeword, wherein the target codeword is different from the respective codeword; and a transmitter configured to transmit the second packet at a second frequency different than the first frequency.
 2. The backscatter tag communication device of claim 1, wherein translating the respective codeword to the target codeword comprises changing a phase of the respective codeword.
 3. The backscatter tag communication device of claim 2, wherein the communication protocol comprises one of ZigBee, WiFi 802.11(g), or WiFi 802.11(n).
 4. The backscatter tag communication device of claim 2, wherein the phase is changed based on a data rate of the transmitter.
 5. The backscatter tag communication device of claim 1, wherein said backscatter tag communication device transmits the second packet during one of a plurality of time slots at random.
 6. The backscatter tag communication device of claim 5, wherein a count of the plurality of time slots is a variable number.
 7. The backscatter tag communication device of claim 1, wherein the first packet has a predefined duration.
 8. The backscatter tag communication device of claim 7, further comprising: an envelope detector configured to detect a duration of the first packet.
 9. The backscatter tag communication device of claim 1, wherein the backscatter tag communication device is configured to multiply an excitation signal defining the first packet with the data that the backscatter tag communication device seeks to transmit to generate a backscatter tag signal defining the second packet.
 10. A method of communication via a backscatter tag, the method comprising: receiving a first packet comprising a first plurality of codewords, the first packet conforming to a communication protocol defining a second plurality of codewords inclusive of the first plurality of codewords, wherein the second plurality of codewords comprises two or more codewords, wherein the first packet is characterized by a first frequency, and wherein each codeword comprises a plurality of bits; generating a second packet by, for each respective codeword of the first plurality of codewords and based on a data that the backscatter tag is invoked to transmit: selecting a target codeword of the second plurality of codewords; and translating the respective codeword to the target codeword, wherein the target codeword is different from the respective codeword; and transmitting the second packet at a second frequency different than the first frequency.
 11. The method of claim 10, wherein translating the respective codeword to the target codeword comprises changing a phase of the respective codeword.
 12. The method of claim 11, wherein the communication protocol comprises one of ZigBee, WiFi 802.11(g), or WiFi 802.11(n).
 13. The method of claim 11, wherein the phase is changed based on a data rate of a transmitter transmitting the second packet.
 14. The method of claim 10, further comprising: transmitting the second packet during one of a plurality of time slots at random.
 15. The method of claim 14, wherein a count of the plurality of time slots is a variable number.
 16. The method of claim 10, wherein the first packet has a predefined duration.
 17. The method of claim 16, wherein the backscatter tag comprises an envelope detector configured to detect a duration of the first packet.
 18. The method of claim 10, further comprising: multiplying an excitation signal defining the first packet with the data that the backscatter tag seeks to transmit to generate a backscatter tag signal defining the second packet. 